This invention relates to stress relieved grinding balls to enhance durability of the balls, particularly in heavy duty grinding environments.
Various technologies are available for manufacturing grinding balls for use in grinding mills, such as in ore crushing, stone crushing and the like. Grinding balls are usually 2.5 to 14 centimeters in diameter depending upon the size of the grinding mill. Balls can be cast from iron using a combination of alloys to develop the desired hard, wear resistant surface. However, the high cost of casting and the high cost of alloys required by this process usually make it prohibitively expensive. More commonly, balls are forged from steel with a selected chemistry and heat treated to give the best combination of wear rate and toughness. It has been found that the useful life of a grinding ball may be improved if it has a hard, tough outer shell usually of martensitic microstructure. The high hardness is required to reduce the erosive wear prevalent in grinding applications. The shell toughness is required to prevent the loss of pieces of the ball by spalling. In addition to shell toughness, the ball requires a core toughness that keeps the entire ball from breaking, particularly in the case of larger balls. Examples of such grinding ball technology are described in Canadian patents 399,994 issued Oct. 14, 1941 and 433,070 issued Feb. 12, 1946.
The ball toughness is directed towards preventing breakage by the ball stresses. This is particularly true with larger balls, usually larger than 7 to 8 cm in diameter. A moderate level of compressive stress in the outer shell which is balanced by tensile stresses in the core help hold the relatively brittle ball steel together and prevent ball breakage. In addition, moderate compressive shell stresses help prevent spalling. High ball stresses, which exceed the tensile strength of the core or the compressive strength of the shell, cause breakage or spalling. Low ball stresses, which allow the surface of the ball to go into tension, can also cause breakage.
Accordingly, this invention provides grinding balls that have the desired wearability and have the desired durability in grinding environments. The advantage of this invention has been surprisingly provided by way of a stress relieving technique for already tempered grinding balls, particularly for larger balls having a hardness of an outer martensitic shell of a hardness greater than 55 and usually 60 to 65 Rockwell C and an inner pearlitic core. Although stress relieving techniques have been used in conjunction with tool steels, this is generally understood by those skilled in the art to perform different functions in view of the high alloy contents and high carbon contents of tool steels. The purpose of stress relieving is to modify the structure of the tool steel so that, for example with tool steels, stress relieving is conducted at relatively high temperatures usually around 500xc2x0 C. In view of the high alloy content, it is generally understood that stress relieving at these high temperatures brings about a change in the characteristic of the tool steel. Conversely, it is generally understood that tempering of carbon and low alloy steel products after the first temper does not bring about any significant changes in the physical characteristics of the product.
In the ore grinding field, applicant has developed a technique for stress relieving grinding rods which present unique heat treating problems because of their overall length usually greater than 10 feet. Quenching of such rods can be achieved in a special quenching chamber where high speed flows of quenching liquid, preferably water, passes along the length of the rod to achieve very rapid quenching of the rod. This type of quenching step greatly enhances the Rockwell hardness of the material. Applicant has found that, stress relieving such rapidly quenched rods, greatly reduces the potential of rod break-up, increases rod toughness and durability of the rod and provides prolonged rod life in a grinding environment.
In accordance with an aspect of the invention, a grinding ball is provided which has a hardened outer shell of tempered martensite wherein the ball has been stress relieved to stabilize the ball against break up and/or spalling.
In accordance with another aspect of the invention, a process for making a grinding ball having a hardened section of tempered martensite wherein said ball has been stress relieved to stabilize said ball against break up and/or spalling, the process comprising
i) reheating a tempered grinding ball having a hardened section of tempered martensite to its previous equalization temperature of its earlier tempering process;
ii) holding the grinding ball at the equalization temperature for a period of time sufficient to relieve partially compressive stresses in the tempered martensite section to develop an outer stress relieved martensitic shell and an inner non-stress relieved martensitic section; and
iii) allowing the reheated stress relieved ball to cool.
In accordance with another aspect of the invention, a grinding ball has a hardened section of tempered martensite wherein the ball has been stress relieved to stabilize the ball against break up and/or spalling by developing an outer stress relieved martensitic shell.